The recent trend of magnetic tape is toward increasing density. Among others, research efforts are concentrated on magnetic tape having a ferromagnetic metal thin film based on cobalt and having nickel or the like added because of its high saturation magnetic flux density and coercivity.
Magnetic tape of this type is prepared by a variety of methods although advantageous methods proposed heretofore are by forming a ferromagnetic metal thin film as a mono-layer film on a non-magnetic substrate by an oblique evaporation technique and by depositing two or more layers to form a multilayer structure.
The non-magnetic substrates used for magnetic tape are generally resin films of polyethylene terephthalate or the like.
Where a magnetic layer is formed on such a resin film by evaporation, moisture and air can penetrate through the resin film to corrode the magnetic layer, degrading electromagnetic properties upon recording and reproducing. Also durability is insufficient.
One solution to this problem is to form a plasma polymerized film of 20 to 270 .ANG. thick on the magnetic layer as disclosed in Japanese Patent Application Kokai (JP-A) Nos. 72653/1984 and 154641/1984.
Also JP-A 219626/1985 discloses to form a plasma polymerized fluoride film on the magnetic layer.
None of these proposals achieved sufficient rust prevention since the film formed as a topcoat film on the magnetic layer could not prevent entry of moisture or the like through the substrate.
Particularly, the plasma polymerized fluoride film has too weak quality to provide durability. It may occur to coat lubricant in order to compensate for such a loss of durability, but the plasma polymerized film repels the coating solvent. Due to the loss of affinity between the plasma polymerized film and the lubricant, a lubricant layer cannot be provided.
Further, JP-A 22420/1986 proposes the use of a plasma polymerized film as both a topcoat film and a backcoat film on the rear surface of the substrate.
However, the provision of such a backcoat film gives no more than the rust preventing effect available with only the topcoat film.
Therefore, these prior art methods are insufficient in shutting off moisture, oxygen or the like and thus in corrosion resistance and durability. In particular, the cupping problem that magnetic tape is transversely warped due to rust generation is rigorous because cupping degrades head touch, causing poor travel performance.